Molybdenum flame spray powder and process

ABSTRACT

The bulk density of a flame spray powder of agglomerates of finely divided molybdenum or molybdenum alloy particles is increased by adding from 0.1 to 0.5 weight percent of finely divided nickel particles to the agglomerates and heating the agglomerates at a temperature of from 1050*C to 1300*C for from 1 to 7 hours, resulting in increased flame spray coating rates due to the increased weight of metal sprayed per unit of time.

United States Patent Port et al.

[ Sept. 23, 1975 MOLYBDENUM FLAME SPRAY POWDER AND PROCESS Inventors: David ,1. Port, Athens; Richard F.

Cheney, Towanda, both of Pa.

GTE Sylvania Incorporated, Stamford, Conn.

Filed: Aug. 21, 1974 Appl. No.: 499,139

Related U.S. Application Data Continuation-impart of Ser. No. 455,610, March 28, 1974. abandoned.

Assignee:

U.S. Cl. 75/0.5 AB; 29/192 CP; 75/O.5 BB; 148/126; 264/7 Int. Cl. B22F 1/02 Field of Search 75/0.5 B, 0.5 BB, 0.5 AB; 29/192 CP; 264/7; 148/126 References Cited UNITED STATES PATENTS 10/1968 Timmons 75/0.5 BB

BULK DENSITY /cu. cm.

3,617,358 11/1971 Dittrich 75/05 B Primary Examiner-W. Stallard Attorney, Agent, or FirmNorman J. OMalley; John C. Fox; Castle Donald R.

[57] ABSTRACT The bulk density of a flame spray powder of agglomerates of finely divided molybdenum or molybdenum alloy particles is increased by adding from 0.1 to 0.5 weight percent of finely divided nickel particles to the agglomerates and heating the agglomerates at a temperature of from 1050C to 1300C for from 1 to 7 hours, resulting in increased flame spray coating rates due to the increased weight of metal sprayed per unit of time.

10 Claims, 4 Drawing Figures SIZE FRACTION -l70 +200 MESH TEMPERATURE '0 FOR 3%HRS.

US Patent Sept. 23,1975 Sheet 1 012 3,907,546

" SIZE FRACTION 3 -|70 +200 MESH BULK DENSITY /cu. cm.

TEMPERATURE c FOR 3%HRS.

INO Ni SIZE FRACTION 2 0.|% Ni -200 +325 MESH 3 0.2%Ni I 4 0.3%Ni

BULK DENSITY -/cu.cm.

TEMPERATURE "C FOR 3% HRS.

US Patent Sept. 23,1975 Sheet 2 of2 3,907,546

SIZE FRACTION NONI 47o +325MESH BULK DENSITY /ClLCm.

TEMPERATURE '0 FOR 3%HRS.

o Ni SIZE FRACTION 2 0.1%m 325 MESH 3 0.2% m 4 0.3%m 5 0.5%m BULK DENSITY -/cu.cm.

TEMPERATURE C FOR 3%HRS.

MOLYBDENUM FLAME SPRAY POWDER AND PROCESS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 455,610, filed Mar. 28, 1974, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to molybdenum and molybdenum alloy flame spray powders, and more particularly to free flowing particle agglomerates of finely divided molybdenum and molybdenum alloy powders having improved bulk densities, and to the process for producing them.

Metallic and ceramic flame spray coatings are frequently applied to various articles to impart properties such as hardness, wear resistance, good lubricity, corrosion resistance, improved electrical properties or perhaps simply to build up a used part which has worn below useable tolerances. Powders for flame spray use are desirably uniform in size and composition, and relatively free flowing. Flowability must be sufficient for the powders to be uniformly transported to and injected into the flame. In general, the finer the powders, the poorer the flow characteristics. Although considerable advances have been made in powder feeding equipment, powders less than about 40 micrometers generally do not flow well enough for general use.

The ceramics and powder metallurgy industry have used various agglomeration methods in order to make free flowing powders of normally nonflowing small diameter powder particles, usually involving use of an organic binder to promote formation of the agglomerates. Because of their larger sizes and relatively lower surface area the agglomerates have improved flow properties. Unfortunately, such agglomerated product also has a lower apparent density than the beginning particulate product. This property is the weight of a given volume of uncompacted, loose powder, and is impor tant in flame spraying in that the weight of the coating being deposited depends on the weight of the volume of powder which the flame gun feeder will accept. In addition, the agglomerated product has a larger mean particle size than the beginning material. This is important in that when considering two materials of comparable size ranges, the one having the smaller mean particle size gives a denser, smoother coating. Strength is often improved with denser coatings and smoother coatings require less finishing by grinding or machining.

Flame spray powders having high apparent densities have been made by atomization of molten material. However, atomization processes are characterized by low yields of particles within the desired size range. Furthermore, powders of refractory material are difficult and costly to produce by atomization techniques primarily because of their high melting points.

SUMMARY OF THE INVENTION Improved flame spray powders of free flowing agglomerates of finely divided molybdenum and molybdenum alloy particles are produced by the addition of from 0.1 to 0.5 weight percent of finely divided nickel particles to the agglomerates, followed by heating the powder to below the melting point of the nickel to ef fect migration of nickel atoms along the surfaces of the agglomerate subparticles in order to at least partially coat the surfaces of the particles with a nickel layer, and to promote densification of the agglomerates. This densification is advantageous in that it leads to increased efficiency in coating operations.

Such heating may also result in some interdiffusion of atoms across the interfaces between the particles and the nickel layers, but will be insufficient to result in complete alloying of the two metals, since that degree of heating would in general result in the sintering together of the agglomerates to form an unuseable mass or cake of material. By incomplete alloying is meant that the nickel atoms are not uniformly dispersed in the molybdenum or alloy particles, but rather exist in measurably higher concentrations at or near the particles surfaces than in the interior portions of the particles.

In accordance with a preferred embodiment, materials in finely divided particulate form (less than about 40 micrometers) are agglomerated such as by spray drying in a slurry with a binder, and classified to obtain a desired particle size range. At this stage the agglomerates are porous. For example, for agglomerates having sizes below mesh bulk densities typically range from about 2.2 to 2.7 grams per cubic centimeter. The agglomerates are then processed as above, resulting in substantial increases in bulk density. For example, for the above particle sizes bulk densities as high as 3.6 grams per cubic centimeter can be achieved for molybdenum particles while substantially retaining agglomerate integrity and thus free flowing characteristics.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph in which bulk density in grams per cubic centimeter is plotted vs heating temperature in degrees Centigrade for 3% hours for 5 different molybdenum flame spray powders containing from O. to 0.5 weight percent nickel, and having a particle size range between 170 and 200 mesh;

FIG. 2 is a graph similar in all respects to the graph of FIG. 1 except for a particle size range between 200 and 325 mesh;

FIG. 3 is a graph similar in all respects to the graph of FIG. 1 except for a size range between 170 and 325 mesh; and

FIG. 4 is a graph similar in all respects to the graph of FIG. 1 except for a size fraction below 325 mesh.

DETAILED DESCRIPTION OF THE INVENTION For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

For purposes of the invention, the term finely divided particles refers to particles exhibiting poor flowability, generally of particle sizes below 40 micrometers.

Where the beginning particle size of the powder is below about 40 micrometers, the flowability of the powder is in general insufficient to permit readily entraining them in a carrier gas and feeding them through the flame spray gun. Thus, such particles must normally first be agglomerated. Such agglomeration may be by any technique known to the art such as forming powder compacts followed by crushing these compacts or mixing the powder with a binder in the presence of moisture. However, agglomeration by spray drying is in general preferred for its flexability and economy of operation on a production scale. The particular conditions under which the slurries are formed and spray dried are well known, and are not a necessary part of this description. A detailed description thereof may be found, for example, in U.S. Pat. No. 3,617,358, issued Nov. 2, 1971.

In addition to the molybdenum powder, there may be added to the agglomerates powder particles of an element which will form a solid solution with molybdenum to thereby increase the hardness of the resulting alloy over that of pure unalloyed molybdenum and thus improve the hardness and wear resistance of flame spray coatings produced from such alloyed powders. Such elements include tungsten, rhenium, chromium and the group VIII elements Fe, Co, Ru, Rh, Pd, Os, Ir and Pt. As is known, hardening occurs because the atoms of the alloying element lock the dislocations in crystal structure, thus requiring more energy to be applied for deformation to occur. Since the alloy is a solid solution, this locking occurs without the creation of additional phases or changes in crystal structure which could result in the loss of important properties. For example, additional phases often are mechanically dislodged from the matrix phase and in a flame sprayed coatings could act as sources of contamination. For example, in an internal combustion engine, where such coatings are used on piston rings, such contamination could cause galling or scoreing of cylinder walls. Also, additional phases are more likely to cause changes in coefficient of friction than are solid solution elements. The solid solution alloying elements may be present in amounts of up to 50 weight percent of the flame spray powder, from to weight percent being preferred. In these amounts such elements exhibit a substantial increase in hardness of the resulting alloy. For example, for a molybdenum 15 percent tungsten alloy coating, a 10 percent increase in hardness, from 462 to 508 DPI-I has been observed.

The desired amount of alloying element and of nickel, from 0.1 to 0.5 weight percent of the flame spray powder, may be added to the agglomerates by mixing the powders with the molybdenum powder prior to agglomeration. In the case of agglomeration by spray drying, the powders may be added to the spray drying slurry along with the molybdenum powder and a suitable binder. Alternatively, or in addition, a salt which is soluble in the slurry liquid and which decomposes to the metal upon heating in a reducing atmosphere such as Ni Cl or ammonium tungstate may be employed, as described in copending patent application Ser. No. 414,976, filed Nov. 12, 1973, and assigned to the present assignee.

The heating of the thus formed agglomerates, while essential to be carried out below the melting point of nickel, (at which temperature substantial sintering together of the agglomerates might occur), may be carried out between 900C and I300C, for from /2 to 7 hours, below which bulk density is not appreciably improved and above which some agglomerate integrity and consequently, some flowability may be lost due to formation of sinter welds between adjacent agglomerates. Based upon these considerations, it is preferred to carry out heating between 1 100C and 1300C for from 3 to 5 hours. The heating atmosphere may be protective, that is, nonoxidizing, or reducing and, except where decomposition of a powder component to the metallic state requires a reducing atmosphere, the atmosphere is chosen based upon convenience.

EXAMPLE I Five test powders are prepared containing 0, 0.1, 0.2, 0.3, and 0.5 percent nickel by adding the appropriate amounts of carbonyl nickel metal powder sifted through 200 mesh (all screen sizes herein are U.S. Standard Sieve) to slurries for spray drying. These slurries have about 65 percent solids concentration with 15 percent of the solids weight being binder and the remainder being molybdenum powder. These slurries are spray dried.

The chamber product recovered from spray drying each different composition is used to investigate the effect of the nickel additions on bulk density when the sintering temperature is varied. Each of the powders are first carefully sifted into four fractions, minus plus 200 mesh, minus 200 plus 325 mesh, minus 170 plus 325 mesh and minus 325 mesh.

Heat treatments on each size fraction are carried out at four different temperatures; 850C, l050C, 1200C, and 1300C, all for 3 hours in a Hydrogen atmospherepThe bulk density measurements of the heat treated powders are made according to ASTM specification B-212-48. Results are presented in graphic form in FIGS. 1-4 of the Drawing. In general the results show a general increase in the bulk densities of the powders with increasing temperature. However, in every size fraction the sample which contains no nickel shows no appreciable increase in high density up to 1200C. In contrast, each of the other samples measured containing various amounts of nickel additions do show appreciable bulk density increases between l050C and 1200C. From 1200C to 1300C the samples containing no nickel do show some bulk density increase, while the samples containing either 0.1 or 0.2 percent nickel show some bulk density decrease. However, the total bulk density change from l050C to 1300C indicates that the 0.1 and 0.2 percent nickel samples show an overall bulk density increase significantly greater than that shown by the sample containing no nickel. For example, in FIG. 1 the bulk density increase for the 0 percent inckel sample is about 12.7 percent between l050C and 1300C. while the bulk density increase for the 0.2 percent nickel sample within the same range is about 21.3 percent. Similarly in FIG. 2 the increase for 0 percent nickel is 4.3 percent while the increases for 0.1 and 0.2 nickel samples are 10.1 and 20.8 percent respectively. In FIG. 3 the overall bulk density increase from l050C to I300C is 12.3 percent for the 0 percent nickel sample, but 18.8 percent for the 0.2 nickel sample. Finally in FIG. 4 the overall increase for the 0 percent nickel sample is 12.3 percent and the overall increase for the 0.2 nickel samplc is about 20 percent. The overall increases for the 0.3 percent and 0.5 percent nickel samples are appreciabley greater appreciably l050C to I300C than are the increases for the remaining samples.

EXAMPLE II A first slurry of combined metal powders is prepared from 21.3 kg. of powdered molybdenum, 3.2 kg of powdered tungsten and 21 g of nickel powder. These powders are added to 7.5 liters of water containing 427 g of Carbowax 6000, a tradename for polyethylene glycol and 107 g of polyvinyl acetate. A second slurry of ammonium tungstate is prepared by adding 9.08 kg. of powdered molybdenum and 10.5 g of Ni to 7.5 liters of ammonium tungstate solution which assays at 276 grams of W0 per liter. These slurries are each spray dried in a commercial spray dryer under the same conditions. The inlet temperature is 265C and the outlet temperature is 130C. The nozzle pressure is 37 psi.

The chamber products are processed further by being subjected to a presintering cycle of 3 /2 hours at 1060C. Both powders exhibit good bulk properties after presintering, as shown by the following results in Table I.

TABLE I SLURRY OF SLURRY IN SIEVE ANALYSIS POWDERS AMMONIUM TUNGSTATE +100 1% 2% +200 34% 34% +325 31% 32% 325 34% 32% -200+325 Fraction Apparent Density 2.03 g/cc 2.2 g/cc Hall Flow 33 sec. 31 sec.-

The powders are then sintered for /2 hour at l200C. Both powders are densified during this step as evidenced by a l6l7 percent increases in their apparent densities. The sieve analyses show general increases in the agglomerate size in both tests. Results are shown in The -200+325 mesh fraction of the powder made from the ammonium tungstate slurry is fired once more at 1200C for /2 hour. An apparent density increase of 0.12 g/cc up to 2.68 g/cc and a Hall Flow time decrease to 24 seconds are achieved.

While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

l. A free flowing flame spray powder comprising agglomerates of powder particles, said agglomerates consisting essentially of subparticles of molybdenum and from 0 to 50 weight percent of a solid solution alloying element selected from the group consisting of tungsten, rhenium, chromium and the Group VIII elements, said subparticles at least partly coated with layers of nickel, at least a portion of the subparticles containing zones of diffused nickel atoms adjacent to at least a portion of the surface areas of contacts of the subparticles with the nickel layers, and at least a portion of the nickel layers containing diffused atoms from the subparticles, the total amount of nickel layers in the agglomerates being from 0.1 to 0.5 weight percent of the agglomerates.

2. The powder of claim 11 having agglomerate sizes below 170 mesh and bulk densities up to 3.6 grams per cubic centimeter.

3. The powder of claim 1 wherein the subparticles consist essentially of from 10 to 20 weight percent tungsten, remainder molybdenum.

4. Process for producing a free flowing flame spray powder comprising:

a. agglomerating a mixture of finely divided particles to produce a free flowing powder of particle agglomerates, the mixture consisting essentially of molybdenum powder particles, powder particles of a solid solution alloying element for molybdenum selected from the group consisting of tungsten, rhenium, chromium and the group VIII elements in the amount of from O to 50 weight percent of the molybdenum powder, and nickel powder particles, the nickel powder being present in an amount of from 0.1 to 0.5 weight percent of molybdenum and alloying powder; and

b. heat treating the powder agglomerates at a temperature below the melting point of nickel to increase the bulk density thereof.

5. Process of claim 3 in which the heat treatment is carried out within the temperature range of from 900C to 1300C for from /2 to 7 hours.

6. Process of claim 5 in which the heat treatment is carried out within the temperature range of from 1 C to 1300C for from 3 to 5 hours.

7. Process of claim 5 in which the agglomerate size of the powder mixture is below mesh.

8. Process of claim 4 in which nickel is present in the powder mixture in the amount of from 0.1 to 0.2 weight percent and the powder agglomerates are heated at a temperature of from 1050C to l200C for from /2 to 5 hours.

9. Process of claim 4 in which the agglomerated powder is formed by spray drying a slurry of finely divided particles in a liquid, the liquid comprising a solution of a binder in a volatile solvent.

' 10. Process of claim 4 in which the particles of the alloying element consist essentially of tungsten and are present in the amount of from 10 to 20 weight percent. l=

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,907,546 DATED September 23, 1975 INVENTOR(S) David J. Port 6: Richard F. Cheney It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 31 Please delete "in high density" and insert in bulk density Co]. 4, lines 57 &

58 Please delete "appreciabley greater D appreciably" and insert appreciably greater from Signed and Scaled this [SE Sixteenth December 1975 A ttest:

RUTH c. MA Anew-"g fg c. MARSHALL DANN CmnmissiW' "flarentx and Trademarks UNITED STATES PATENT OFFICE PATENT NO. 3,907,54 DAT September 23, 1975 INVENTOR(S) David J. Port & Richard F. Cheney It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 31 Please delete "in high density" and insert in bulk density Co]. 4, lines 57 8:

58 Please delete "appreciabley greater appreciably" and insert appreciably greater from Signed and Scaled this [SEAL] sixteenth D3) of December 1975 Arrest:

RUTH C. MA Arrestin 007:3 MARSHALL DANN C ummissiuner uj'ParenIs and Trad 

1. A FREE FLOWING FLAME SPRAY POWDER COMPRISING AGGLOMERATES OF POWDER PARTICLES, SAID AGGLOMERATES CONSISTING ESSENTIALLY OF SUBPARTICLES OF MOLYBDENUM AND FROM 0 TO 50 WEIGHT PERCENT OF A SOLID SOLUTION ALLOYING ELEMENT SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, RHENIUM, CHROMIUM AND THE GROUP VIII ELEMENTS, SAID SUBPARTICLES AT LEAST PARTLY COATED WITH LAYERS OF NICKEL, AT LEAST A PORTION OF THE SUBPARTICLES CONTAINING ZONES OF DIFFUSED NICKEL ATOMS ADJACENT TO AT LEAST A PORTION OF THE SURFACE AREAS OF CONTACTS OF THE SUBPARTICLES WITH THE NICKEL LAYERS, AND AT LEAST A PORTION OF THE NICKEL LAYERS CONTAINING DIFFUSED ATOMS FROM THE SUBPARTICLES, THE TOTAL AMOUNT OF NICKEL LAYERS IN THE AGGLOMERATES BEING FROM 0.1 TO 0.5 WEIGHT PERCENT OF THE AGGLOMERATES.
 2. The powder of claim 1 having agglomerate sizes below 170 mesh and bulk densities up to 3.6 grams per cubic centimeter.
 3. The powder of claim 1 wherein the subparticles consist essentially of from 10 to 20 weight percent tungsten, remainder molybdenum.
 4. Process for producing a free flowing flame spray powder comprising: a. agglomerating a mixture of finely divided particles to produce a free flowing powder of particle agglomerates, the mixture consisting essentially of molybdenum powder particles, powder particles of a solid solution alloying element for molybdenum selected from the group consisting of tungsten, rhenium, chromium and the group VIII elements in the amount of from 0 to 50 weight percent of the molybdenum powder, and nickel powder particles, the nickel powder being present in an amount of from 0.1 to 0.5 weight percent of molybdenum and alloying powder; and b. heat treating the powder agglomerates at a temperature below the melting point of nickel to increase the bulk density thereof.
 5. Process of claim 3 in which the heat treatment is carried out within the temperature range of from 900*C to 1300*C for from 1/2 to 7 hours.
 6. Process of claim 5 in which the heat treatment is carried out within the temperature range of from 1100*C to 1300*C for from 3 to 5 hours.
 7. Process of claim 5 in which the agglomerate size of the powder mixture is below 170 mesh.
 8. Process of claim 4 in which nickel is present in the powder mixture in the amount of from 0.1 to 0.2 weight percent and the powder agglomerates are heated at a temperature of from 1050*C to 1200*C for from 1/2 to 5 hours.
 9. Process oF claim 4 in which the agglomerated powder is formed by spray drying a slurry of finely divided particles in a liquid, the liquid comprising a solution of a binder in a volatile solvent.
 10. Process of claim 4 in which the particles of the alloying element consist essentially of tungsten and are present in the amount of from 10 to 20 weight percent. 